You need a compass that can be boxed (i.e. has adjustable corrective magnets to counteract the iron in the vicinity of the compass). Aircraft compasses would be one option, though they may have some trouble because most aircraft metal is aluminum and not ferrous. Many marine compasses would likely work very nicely as well.
If you're moving, a GPS can tell you what direction you're heading in with sufficient accuracy for most things. This is not quite the same thing as the actual heading of the motorhome—it's what direction you've been traveling in, not what direction you're currently pointed, but most of the time that distinction doesn't matter much. Backing up is one (big) exception.
